Add support for wxStatusBar::AddFieldControl()

[KeyWorksRW]

Description:

wxWidgets 3.3 adds wxStatusBar::AddFieldControl() making it possible to add controls to the various status bar fields. Support for this should be added, with conditionals for 3.3.

Note that adding any widget as a child isn't going to work, since the index into the field needs to be specified. I.e., a status bar with 4 fields might want the third field to be a checkbox, and normal fields for position 1, 2, and 4.